1. Field of the Invention
The present invention relates to an aluminum nitride circuit board and a method of producing same, and more particularly, to an aluminum nitride circuit board used in a semiconductor device assembly, etc., and a method of producing same.
2. Related Art
Recently, as the operating speed and output of semiconductor elements increase and the semiconductor elements become more highly integrated, in order to put these elements into practical use, it is important to improve the heat radiating properties, reduce the resistance of the conductor portion, and reduce the dielectric constant of the base portion of the circuit board which supports the elements.
Aluminum nitride ceramics is used as a practical material to solve the aforementioned technical problems. As compared with alumina ceramics that have been used for this purpose, aluminum nitride ceramic has a high thermal conductivity, a low dielectric constant, and a low thermal expansion coefficient. Accordingly, aluminum nitride ceramics is practically suitable for use with large-sized silicon elements of high output.
Therefore, a circuit board, the base plate of which is made of aluminum nitride ceramics, has been developed for assembling semiconductor devices.
In the case where a wiring pattern is formed on or in a ceramic circuit board, it is possible to form a wiring pattern on an outer surface of the base plate by means of a conventional thin film forming method or metalizing after firing. However, concerning a conductor portion formed inside the circuit board, the internal conductor wiring must be provided before firing and metalized in a co-firing process.
Conventionally, tungsten is used as a metalizing material in the co-firing process. However, the specific resistance of tungsten is higher than that of copper or gold. Therefore, when tungsten is used for signal transmission, power is lost, and further the transmission speed is lowered. Accordingly, tungsten is not suitable for the wiring material.
In order to solve the above problems, the inventors have proposed a method for producing aluminum nitride circuit boards in which a low resistance material such as copper, gold and silver is used as a metalizing material instead of tungsten (disclosed in Japanese Unexamined Patent Publication (Kokai) No. Hei. 2-197189). According to this method, green sheets, on which a predetermined wiring pattern and via hole connecting portions are formed using a conductor paste of copper, gold, silver, or the like, are laminated and protective green sheets are further layered on outer surfaces of the laminated body, so that the wiring patterns are covered with the protective green sheets. Then, the laminated body is fired. After the firing, the layers covering the surfaces of the laminated body is ground so that the wiring patterns are exposed to the outside and thus a circuit board is obtained.
The aforementioned aluminum nitride circuit board, in which a low resistance material such as copper, gold and silver is used for the metalizing material, has excellent electrical characteristics. However, according to the producing method of the prior art, after the aluminum nitride layered body has been fired, the protective, aluminum nitride green sheet must be ground in order to expose the wiring pattern. This grinding work takes much time. Accordingly, the product cost is raised due to the grinding or polishing work. In the case where a plane grinder is used for grinding, a large amount of shavings are produced, and further a large amount of gas is generated, which causes some operational problems.
In the aforementioned producing method, the melting point of a conductor metal such as copper and gold used for the wiring pattern is lower than the firing temperature of aluminum nitride ceramics. Therefore, the following problems may be encountered: When a green sheet is fired, a conductor paste is melted, so that a volume of the conductor paste is reduced, and vacancies are generated in the conductor portion which result in the breakage of wire.
In the case of the aforementioned method for producing circuit boards (disclosed in Japanese Unexamined Patent Publication (Kokai) No. Hei. 2-197189), copper is most preferably used for the conductor metal. In the case where copper is used for the conductor metal, it can be preferably co-fired at a temperature not less than 1,800.degree. C., and the sintering additive is hardly mixed with the conductor portion. On the other hand, in the case where silver is used for the conductor metal, it is difficult to co-fire at a firing temperature exceeding 1,700.degree. C., and in the case of gold, it was recognized that the sintering additive component was mixed with the conductor portion.
Copper can be preferably used for the conductor material in the process of co-firing. However, in the case of copper, the sintering additive is hardly mixed into the conductor, and material is hardly moved through an interface between the conductor portion and the ceramics. Therefore, an adhesive force between the conductor portion and the ceramics is mainly generated by a mechanical action caused by the interlock mechanism, so that the connection between the conductor portion and the ceramic is generally weak. Consequently, the adhesive force between the conductor portion and the ceramics is weak.
As a result, in the case where copper is used for the conductor material, the following problems arise: the adhesive strength of the inner conductor portion which has been exposed onto the sintered body surface by grinding is weak; and in the case of a circuit board on which a large number of via hole portions are formed, the circuit board tends to crack on the interface between the via hole portions and the ceramic. Further, the wetting properties between melted copper and aluminum nitride ceramics are not good when copper is melted in the process of firing. Therefore, the melted copper is separated into several portions in he middle of the wiring, which causes breaking of the wire (i.e., electrical open).